Heretofore it has been common practice to use chlorinated solvents such as trichlorethylene, perchloroethylene, 1,1,1-trichloroethane, methylene chloride and trichlorotrifluorethane and mixtures of trichlorotrifluoroethane with substances such as toluol, surfactants, alcohols such as methyl alcohol and inhibitors for removing oil and oil-like contaminants from metal. Although effective for cleaning metal, such solvents present significant toxicological and environmental problems in their storage, use and discard.
Non-chlorinated solvents such as alcohols, toluol, methyl ethyl ketone, mineral spirits and kerosene have also been used in the past to remove oil and oil-like contaminants from metal but likewise present significant toxicological, storage and environmental problems as well as flammability and explosion problems in their storage, use and discard. Both chlorinated and non-chlorinated solvents characteristically are unable to effectively remove carbonaceous soils and water spots from metal surfaces.
Water-based cleaners have also been employed for many years for cleaning metal parts. But such cleaners characteristically have not been as effective a cleaner as the solvents previously described and tend to leave, or are designed to leave residue deposits on the metal surfaces which inhibit painting, welding and/or effective bonding of other materials to the metal surface. Or, if the surface is sufficiently clean for painting or bonding and the like, corrosion such as rust on ferrous metal parts can occur in seconds which may, in some cases, render the part useless.
Due to the tendency of water to promote corrosion, corrosion inhibitors such as sodium or potassium sulfonates, sodium nitrite, or barium napthiate are commonly added to water and cleaners. Such inhibitors are characteristically of a residue type that provide a polar or non-polar film on the surface being cleaned to prevent oxygen from attacking the surface but which also can be detrimental to subsequent processes on the surface such as painting, welding or the bonding of wear resistant and coatings such as titanium nitride.
There has therefore existed a need to provide a process for removing oil and oil-like contaminants from metal that is water-based and does not employ chlorinated or non-chlorinated solvents such as previously described yet which is capable of providing an essentially residue free surface as well as minimizing or preventing rusting of ferrous metals at least for a time sufficient to enable some subsequent process scheduled therefore.
It has been discovered that such process can be provided where the metal part is washed with an alkaline soap and water solution and rinsed with both deionized water and a mixture of deionized water and morpholine prior to drying according to prescribed schedules.
Although it is known that deionized water is corrosive to ferrous metal, it has been discovered that such can be employed to advantage in the process of the invention by controlling the exposure time of the metal part to the deionized water and that by doing so water spots are essentially eliminated in rinsing processes using deionized water whereas such is characteristically not the case with ordinary tap water which may, by leaving residue deposits, interfere with welding and/or effective adherance of coatings to the metal surface.
Contrary to the teaching of the prior art, it has been discovered that dilute morpholine--deionized water solutions are not corrosive to metal and that evaporating the solutions at about 200.degree. F. does not leave a residue deposit which could interfer with subsequent coating processes.
Aqueous amine solutions, such as a morpholine-water solutions, have been used in the past for passivating steel in preparation for application of non-aqueous protective coatings. An example of such is disclosed in U.S. Pat. No. 4,590,100, the disclosure of which is incorporated herein by reference. The morpholine however is mixed with ordinary water which would tend to water spot and the amine is chosen primarily to provide reaction sites that would chemically bond to selected materials used for the coating. Another example of a use of morpholine for rectifying chlorinated hydrocarbon deposits on copper is disclosed in U.S. Pat. No. 4,080,393, the disclosure of which is incorporated herein by reference. Again however the morpholine is mixed with ordinary water which is also used for rinsing which would promote water spotting which is a detrimental to welding and/or bonding many materials to the metal surfaces.
According to the "Encyclopedia of Chemical Technology", John Wiley and Sons, Volumes 2 and 21 (1983), morpholine is classified as an industrial solvent that is slightly toxic, requiring large amounts be taken orally to be serious and, in undiluted form, is irritating to the skin and breathing fumes in closed places should be avoided. Morpholine is classified as being infinitely soluble in water and is known chemically as either tetrahydro-1, 4 oxazine or diethyleneimide oxide having an aromatic ring structure with nitrogen and oxygen in two of the carbon positions.
In view of such, the use of deionized water, morpholine and deionized water, and alkaline soap and water solutions in prescribed schedules has been found to remove oil and oil-like contaminants as well as other contaminants soluble therein from ferrous and non-ferrous metal surfaces that are essentially residue and water spot free and which is particularly advantageous for preparing metal surfaces for welding and/or to which coating(s) are to be adhered.